Semiconductor light emitting device

ABSTRACT

According to one embodiment, a semiconductor light emitting device includes an n-type semiconductor layer, a p-type semiconductor layer, a light emitting portion, a first layer, a second layer, and an intermediate layer. The semiconductor layers include nitride semiconductor. The light emitting portion is provided between the n-type semiconductor layer and the p-type semiconductor layer and includes a quantum well layer. The first layer is provided between the light emitting portion and the p-type semiconductor layer and includes Al X1 Ga 1-x1 N having first Al composition ratio x 1 . The second layer is provided between the first layer and the p-type semiconductor layer and includes Al x2 Ga 1-x2 N having second Al composition ratio x 2  higher than the first Al composition ratio x 1 . The intermediate layer is provided between the first layer and the light emitting portion and has a thickness not smaller than 3 nanometers and not larger than 8 nanometers and includes In z1 Ga 1-z1 N (0≦z1&lt;1).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/934,391filed Jul. 3, 2013, which is a divisional of U.S. application Ser. No.12/874,510 filed Sep. 2, 2010, and is based upon and claims the benefitof priority from the prior Japanese Patent Application No. 2010-031457,filed on Feb. 16, 2010; the entire contents of each of which areincorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor lightemitting device.

BACKGROUND

Using nitride-based III-V group compound semiconductors such as galliumnitride (GaN), there have been developed semiconductor light emittingdevices such as high-intensity ultraviolet to blue and green lightemitting diodes (LED) and blue-violet to blue and green laser diodes(LD).

In order to achieve a higher efficiency in an LED, it is important toimprove crystallinity of a GaN-based semiconductor, to reduce defectsand non-radiative recombination centers, and to increase an internalquantum efficiency inside a crystal. It is also important how torecombine electrons and holes for light emission in an active layer.Electrons are likely to overflow from the active layer. On the otherhand, holes are likely to have a low carrier density because anactivation ratio of a p-type impurity is low.

JP-B 3446660 proposes the configuration provided with a cap layerbetween an active layer and a p-type clad layer, the cap layer includinga layer which is grown using an N₂ gas for preventing the active layerfrom being separated and a layer which is grown using an H₂ gas forforming a potential barrier.

However, this technique still has room for improving a light emittingefficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the configuration ofa semiconductor light emitting device;

FIG. 2 is a schematic cross-sectional view showing part of thesemiconductor light emitting device;

FIG. 3 is a schematic view showing characteristics of the semiconductorlight emitting device;

FIG. 4 is a schematic view showing characteristics of the semiconductorlight emitting device;

FIG. 5 is a schematic view showing characteristics of a semiconductorlight emitting device of a comparative example;

FIG. 6 is a schematic view showing characteristics of a semiconductorlight emitting device of a comparative example;

FIG. 7 is a graph showing characteristics of semiconductor lightemitting devices;

FIG. 8A to FIG. 8C are schematic views showing the simulation results ofthe characteristics of the semiconductor light emitting devices;

FIG. 9A to FIG. 9C are graphs showing characteristics of thesemiconductor light emitting devices; and

FIG. 10A to FIG. 10C are schematic views showing characteristics ofsemiconductor light emitting devices.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor light emittingdevice includes an n-type semiconductor layer, a p-type semiconductorlayer, a light emitting portion, a first layer, a second layer, and anintermediate layer. The n-type semiconductor layer includes a nitridesemiconductor. The p-type semiconductor layer includes a nitridesemiconductor. The light emitting portion is provided between the n-typesemiconductor layer and the p-type semiconductor layer and includes aquantum well layer. The first layer is provided between the lightemitting portion and the p-type semiconductor layer. The first layerincludes Al_(x1)Ga_(1-x1)N, where first Al composition ratio x1 is anatomic ratio of Al among group III elements. The second layer isprovided between the first layer and the p-type semiconductor layer. Thesecond layer includes Al_(x2)Ga_(1-x2)N, where second Al compositionratio x2 is an atomic ratio of Al among group III elements. The secondAl composition ratio x2 is higher than the first Al composition ratiox1. The intermediate layer is provided between the first layer and thelight emitting portion. The intermediate layer has a thickness notsmaller than 3 nanometers and not larger than 8 nanometers. Theintermediate layer includes In_(z1)Ga_(1-z1)N, where In compositionratio z1 is an atomic ratio of In among group III elements. The Incomposition ratio z1 is not lower than 0 and lower than 1.

Hereinafter, an embodiment of the invention is described in detail withreference to the drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thickness and width of portions, the proportional coefficients ofsizes among portions, etc., are not necessarily the same as the actualvalues thereof. Further, the dimensions and proportional coefficientsmay be illustrated differently among drawings, even for identicalportions.

In the specification of the application and the drawings, componentssimilar to those described in regard to a drawing thereinabove aremarked with like reference numerals, and a detailed description isomitted as appropriate.

FIG. 1 is a schematic cross-sectional view illustrating theconfiguration of a semiconductor light emitting device according to anembodiment of the invention.

FIG. 2 is a schematic cross-sectional view illustrating theconfiguration of part of the semiconductor light emitting deviceaccording to the embodiment of the invention.

As shown in FIG. 1, a semiconductor light emitting device 110 accordingto the embodiment of the invention includes: an n-type semiconductorlayer 10 including a nitride semiconductor; a p-type semiconductor layer20 including a nitride semiconductor; a light emitting portion 30provided between the n-type semiconductor layer 10 and the p-typesemiconductor layer 20 and including a quantum well layer; a first layer41; a second layer 42; and an intermediate layer 40 m.

As illustrated in FIG. 1, the n-type semiconductor layer 10, the lightemitting portion 30, the intermediate layer 40 m, the first layer 41,the second layer 42, and the p-type semiconductor layer 20 are stackedalong a Z-axis direction.

The first layer 41 is provided between the light emitting portion 30 andthe p-type semiconductor layer 20 and includes AlGaN having a first Alcomposition ratio x1. That is, the first layer 41 includesAl_(x1)Ga_(1-x1)N, where the first Al composition ratio x1 is an atomicratio of Al among group III elements. The first Al composition ratio x1is higher than 0 and lower than 1. More specifically, the first Alcomposition ratio is not lower than 0.001 and not higher than 0.05, forexample.

The second layer 42 is provided between the first layer 41 and thep-type semiconductor layer 20 and includes AlGaN having a second Alcomposition ratio x2 higher than the first Al composition ratio x1. Thatis, the second layer 42 includes Al_(x2)Ga_(1-x2)N, where the second Alcomposition ratio x2 is an atomic ratio of Al among group III elements.The second Al composition ratio x2 is higher than 0 and lower than 1 andis higher than the first Al composition ratio x1. More specifically, thesecond Al composition ratio x2 is not lower than 0.1 and not higher than0.2, for example.

The Al composition ratio (the first Al composition ratio x1, the secondAl composition ratio x2, etc.) means a ratio (atomic ratio) of Al amongthe group III elements.

The intermediate layer 40 m is provided between the first layer 41 andthe light emitting portion 30. The thickness of the intermediate layer40 m is not smaller than 3 nm (nanometers) and not larger than 8 nm. Theintermediate layer 40 m includes In_(z1)Ga_(1-z1)N (0≦z1<1). That is,The intermediate layer 40 m includes In_(z1)Ga_(1-z1)N, where Incomposition ratio z1 is an atomic ratio of In among group III elements.The In composition ratio z1 is not lower than 0 and lower than 1. Inother words, the intermediate layer 40 m does not substantially includeAl. For example, the intermediate layer 40 m includes GaN or InGaN.

As shown in FIG. 2, the light emitting portion 30 includes multiplebarrier layers 31 and multiple well layers 32 (quantum well layers)alternately stacked along the Z-axis direction described above. In otherwords, the light emitting portion 30 includes the multiple barrierlayers 31 alternately stacked along the z-axis direction, and the welllayers 32 provided between each of the multiple barrier layers 31.

As shown in FIG. 2, in the embodiment, it is assumed that the barrierlayer 31 (the barrier layer 31 which is the closest to the n-typesemiconductor layer 10 among the multiple barrier layers 31) instead ofthe well layer 32 is in contact with the n-type semiconductor layer 10.It is also assumed that the well layer 32 is provided on a p-typesemiconductor layer 20 side of the light emitting device 30. In otherwords, it is assumed that the well layer 32 (the well layer 32 which isthe closest to the p-type semiconductor layer 20 among the multiple welllayers 32) is in contact with the intermediate layer 40 m.

As illustrated in FIG. 1, the n-type semiconductor layer 10 may include,for example, an n-type GaN layer 11 and an n-type guide layer 12provided between the n-type GaN layer 11 and the light emitting portion30. The n-type guide layer 12 is based on GaN or InGaN, doped with ann-type impurity such as Si, for example.

The p-type semiconductor layer 20 may include, for example, a p-type GaNcontact layer 22 and a p-type GaN layer 21 provided between the p-typeGaN contact layer 22 and the second layer 42. The p-type semiconductorlayer 20 includes a p-type impurity such as Mg which is doped thereintowith a high concentration. The concentration of the p-type impurityincluded in the p-type GaN semiconductor layer 21 is lower than theconcentration of a p-type impurity included in the p-type GaN contactlayer 22.

As shown in FIG. 1, the semiconductor light emitting device 110according to the embodiment may include a substrate 5 made of, forexample, sapphire, a buffer layer 6 provided thereon, an n-type GaNlayer 11 provided on the buffer layer 6, and an n-type guide layer 12provided on the n-type GaN layer 11. The n-type GaN layer 11 and then-type guide layer 12 correspond to the n-type semiconductor layer 10.

Then, the light emitting portion 30 (the barrier layers 31 and the welllayers 32) is provided on the n-type guide layer 12. The intermediatelayer 40 m is provided on the light emitting portion 30. The first layer41 is provided on the intermediate layer 40 m. The second layer 42 isprovided on the first layer 41.

The p-type GaN layer 21 is provided on the second layer 42. The p-typeGaN contact layer 22 is provided on the p-type GaN layer 21. The p-typeGaN layer 21 and the p-type GaN contact layer 22 correspond to thep-type semiconductor layer 20.

On a first major surface on the p-type semiconductor layer 20 side ofthe stacked structural body having the configuration as described above,part of the n-type semiconductor layer 10, the light emitting portion30, the intermediate layer 40 m, the first layer 41, the second layer42, and the p-type semiconductor layer 20 are partially removed and,thus, the n-type semiconductor layer 10 is exposed on the first majorsurface side. An n-side electrode 71 is provided in contact with theexposed n-type semiconductor layer 10, and a p-side electrode 81 isprovided in contact with the p-type semiconductor layer 20.

The above-described semiconductor light emitting device 110 isfabricated, for example, as follows.

First, a buffer layer 6 is formed on a substrate 5 made of sapphire, andthe crystal of an n-type GaN layer 11 doped with an n-type impurity isgrown. The thickness of the n-type GaN layer 11 is approximately 4 μm(micrometers), for example.

For the crystal growth, an MOCVD (Metal Organic Chemical VaporDeposition) method is used, for example. In addition to this method, anMBE (Molecular Beam Epitaxy) method and the like may be used for thecrystal growth. The n-type impurity may include various elements such asSi, Ge, and Sn. In the specific example, Si is used. The doping amountof Si is approximately 2×10¹⁸ cm⁻³, for example. The substrate 5 may bebased on various materials such as GaN, SiC, Si, and GaAs in addition tosapphire.

After that, the crystal of an n-type guide layer 12 is grown on then-type GaN layer 11. The n-type guide layer 12 is based on GaN dopedwith an n-type impurity of approximately 1×10¹⁸ cm⁻³, for example. Thethickness of the n-type guide layer 12 is approximately 0.1 μm, forexample.

Temperatures of growing the n-type GaN layer 11 and the n-type guidelayer 12 are 1000° C. to 1100° C., for example.

As the n-type guide layer 12, In_(0.01)Ga_(0.99)N may be used, inaddition to GaN. A temperature used for growing the n-type guide layer12 in the case of using In_(0.01)Ga_(0.99)N is 700° C. to 800° C., forexample. The thickness of the n-type guide layer 12 may be 0.1 μm evenwhen In_(0.01)Ga_(0.99)N is used for the n-type guide layer 12.

After that, the light emitting portion 30 is formed on the n-type guidelayer 12. For example, the barrier layer 31 includingIn_(0.01)Ga_(0.99)N and the well layer 32 including undopedIn_(0.15)Ga_(0.85)N are alternately stacked in pair for 8 times to forman MQW (Multiple Quantum Well) structure. The thickness of each barrierlayer 31 is set to be 5.0 nm, for example. The thickness of each welllayer 32 is set to be 2.5 nm, for example. The growing temperatures ofthe barrier layer 31 and the well layer 32 are 700° C. to 800° C., forexample. An n-type impurity of approximately 1×10¹⁸ cm⁻³ may be dopedinto the barrier layer 31, or the barrier layer 31 may be an undopedlayer.

The intermediate layer 40 m including, for example, GaN is grown on thelight emitting portion 30. The thickness of the intermediate layer 40 mis set to be not smaller than 3 nm and not larger than 8 nm. In thisexample, the thickness of the intermediate layer 40 m is set to be 5 nm.

The first layer 41 is grown on the intermediate layer 40 m. The firstlayer 41 is based on Al_(0.05)Ga_(0.95)N, for example. In other words, afirst Al composition ratio x1 is 0.05. The thickness of the first layer41 is 5 nm, for example. The first Al composition ratio x1 issubstantially 0.05 (plus minus 20%) and the thickness of the first layer41 is substantially 5 nanometers (plus minus 20%).

The second layer 42 is grown on the first layer 41. The second layer 42is based on Al_(0.20)Ga_(0.80)N. In other words, a second Al compositionratio x2 is 0.20. The thickness of the second layer 42 is 5 nm, forexample. The second Al composition ratio x2 is substantially 0.20 (plusminus 20%) and the thickness of the second layer 42 is substantially 5nanometers (plus minus 20%). In this specific example, the second layer42 is based on Al_(0.02)Ga_(0.80)N with Mg of approximately 4×10¹⁹ cm⁻³.On the other hand, the first layer 41 is based on Al_(0.05)Ga_(0.95)Nbeing undoped with Mg. As a result, the concentration of the p-typeimpurity in the second layer 42 is higher than that in the first layer41.

The p-type GaN layer 21 is grown on the second layer 42. The p-type GaNlayer 21 includes Mg doped with a concentration of approximately 1×10¹⁹cm⁻³. The thickness of the p-type GaN layer 21 is approximately 50 nm,for example. The growing temperature of the p-type GaN layer 21 is 1000°C. to 1100° C., for example.

The p-type GaN contact layer 22 is grown on the p-type GaN layer 21. Thep-type GaN contact layer 22 includes Mg doped with a concentration of1×10²⁰ cm⁻³. The thickness of the p-type GaN contact layer 22 isapproximately 60 nm, for example.

The following device processes are performed on the wafer on which theabove-described crystal growth is sequentially performed.

A p-side electrode 81 including, for example, indium tin oxide (ITO) isformed on the p-type GaN contact layer 22. The thickness of the ITO is0.2 μm, for example. An Au film with a thickness of, for example, 1.0 μmis formed as a p-side pad electrode 82 on part of the ITO.

After the p-side electrode 81 (and the p-side pad electrode 82) isformed, dry etching is performed on part of the stacked structural bodydescribed above to expose part of the n-type GaN layer 11. Then, ann-side electrode 71 is formed in a portion where the n-type GaN layer 11is exposed. The n-side electrode 71 is based on a compound film oftitanium/platinum/gold (Ti/Pt/Au), for example. The thickness of this Tifilm is approximately 0.5 μm, for example. The thickness of the Pt filmis approximately 0.05 μm, for example. The thickness of the Au film is1.0 μm, for example.

The semiconductor light emitting device 110 is fabricated in this way.

In a semiconductor light emitting device such as an LED, electrons arelikely to overflow from the light emitting portion 30. On the otherhand, because an activation efficiency of a p-type impurity is lower, acarrier density of holes is likely to be lowered. Moreover, in a nitridesemiconductor layer, an effective weight of holes is large. Accordingly,a diffusion length of holes is likely to be small. Thus, an improvementof an injection efficiency of holes to the active layer is required.

According to the results of various experiments carried out by theinventors of this application, it has been found that light emission ofa quantum well layer on the p-type semiconductor layer 20 side isdominant in the case of a GaN-based LED in which the quantum wellstructure including In is used for an active layer. In addition, inorder to increase an injection efficiency of holes to an active layer (alight emitting portion 30), it is preferable that an electron overflowsuppressing layer including AlGaN is brought closer to the active layerin terms of a balance with the diffusion length of holes. However, ithas been found that when the electron overflow suppressing layer isbrought excessively closer to the active layer, the influence ofspontaneous polarization of AlGaN is exerted on the quantum well layerincluding In; and therefore, the quantum efficiency decreases contraryto expectation.

The configuration of the embodiment is established based on thesefindings.

In the semiconductor light emitting device 110 according to theembodiment, the second layer 42 is an AlGaN layer whose Al compositionratio is high and functions as an electron overflow suppressing layer tocontrol overflow of electrons.

In order to increase the effects of controlling the overflow ofelectrons, it is preferable that an Al composition ratio in the electronoverflow suppressing layer is high. On the other hand, when the Alcomposition ratio in the electron overflow suppressing layer isexcessively high, an energy band of the well layer 32 adjacent to thep-type semiconductor layer 20 is bent by the influence of spontaneouspolarization. As a result, the light emitting efficiency decreases.

In the semiconductor light emitting device 110 according to theembodiment, the intermediate layer 40 m which does not substantiallyinclude Al, the first layer 41 whose Al composition ratio is low, andthe second layer 42 whose Al composition ratio is high are providedbetween the light emitting portion 30 and the p-type semiconductor layer20 in this order from the light emitting portion 30 toward the p-typesemiconductor layer 20.

FIG. 3 is a schematic view illustrating characteristics of thesemiconductor light emitting device according to the embodiment of theinvention.

More specifically, FIG. 3 illustrates changes of the Al compositionratio Ax in the semiconductor layers from the light emitting portion 30to the p-type semiconductor layer 20 in the semiconductor light emittingdevice 110. In FIG. 3, the horizontal axis represents the Z-axisdirection (stacked direction) and the vertical axis represents an Alcomposition ratio Ax in the semiconductor layers.

As shown in FIG. 3, at an end (an end on a p-type semiconductor layer 20side) of the light emitting portion 30, the well layer 32 is provided,and an Al composition ratio Ax in the well layer 32 is substantially 0.The Al composition ratio Ax is also substantially 0 in the intermediatelayer 40 m. The Al composition ratio Ax in the first layer 41 is a firstAl composition ratio x1, which is not lower than 0.001 and not higherthan 0.5, for example. The Al composition ratio Ax in the second layer42 is a second Al composition ratio x2, which is not lower than 0.1 andnot higher than 0.2, for example.

In this manner, in the semiconductor light emitting device 110, the Alcomposition ratios Ax of the intermediate layer 40 m, the first layer41, and the second layer 42 provided between the light emitting portion30 and the p-type semiconductor layer 20 are increased along a directionfrom the light emitting portion 30 toward the p-type semiconductor layer20. The intermediate layer 40 m, i.e., a semiconductor layer in contactwith the well layer 32 of the light emitting portion 30, does notsubstantially include Al.

With this configuration, the Al composition ratio Ax becomes high on aside closer to the p-type semiconductor layer 20 between the lightemitting portion 30 and the p-type semiconductor layer 20. Accordingly,the effects of the electron overflow suppressing can be sufficientlyobtained. In addition, the side closer to the light emitting portion 30does not substantially include Al. Accordingly, an adverse influence onthe energy band characteristics of the well layer 32 due to thespontaneous polarization in the AlGaN layer including Al is suppressed.

FIG. 4 is a schematic view illustrating characteristics of thesemiconductor light emitting device according to the embodiment of theinvention.

More specifically, FIG. 4 schematically illustrates the energy bandcharacteristics of the semiconductor layers from the light emittingportion 30 to the p-type semiconductor layer 20 in the semiconductorlight emitting device 110. In FIG. 4, the horizontal axis represents theZ-axis direction (stacked direction) and the vertical axis represents anenergy Eg. FIG. 4 illustrates a state of a conduction band CB andcharacteristics of valence band VB.

As shown in FIG. 4, a desired energy band characteristic can be realizedfrom the light emitting portion 30 to the p-type semiconductor layer 20in the semiconductor light emitting device 110.

In other words, the energy characteristics of the well layer 32 which isthe closest to the p-type semiconductor layer 20 is substantially thesame as those of the other well layers 32. In other words, the energycharacteristics of the well layer 32 which is the closest to the p-typesemiconductor layer 20 are substantially not influenced by the secondlayer 42 whose Al composition ratio Ax is high.

As a result, the overflow of electrons is reduced and the injectionefficiency of holes to the active layer can be increased. Thus, thelight emitting efficiency is improved.

FIG. 5 is a schematic view illustrating characteristics of asemiconductor light emitting device of a comparative example.

More specifically, FIG. 5 illustrates changes of Al composition ratiosAx in semiconductor layers from a light emitting device 30 to a p-typesemiconductor layer 20 in a semiconductor light emitting device 119 ofthe comparative example.

FIG. 6 is a schematic view illustrating characteristics of thesemiconductor light emitting device of the comparative example.

More specifically, FIG. 6 schematically illustrates the energy bandcharacteristics in the semiconductor layers from the light emittingportion 30 to the p-type semiconductor layer 20 in the semiconductorlight emitting device 119 of the comparative example.

As shown in FIG. 5, the semiconductor light emitting device 119 of thecomparative example includes a second layer 42 which is adjacent to awell layer 32 of a light emitting portion 30 and which has a high Alcomposition ratio.

In this case, as shown in FIG. 6, the energy characteristics of the welllayer 32 which is the closest to the p-type semiconductor layer 20 aredifferent from the energy characteristics of the other well layers 32.In other words, the energy band of the well layer 32 which is theclosest to the p-type semiconductor layer 20 is bent due to thespontaneous polarization in the second layer 42 whose Al compositionratio is high. Thus, the efficiency decreases in the semiconductor lightemitting device 119 of the comparative example.

In contrast, in the semiconductor light emitting device 110, theintermediate layer 40 m and the first layer 41 are provided, thereby,the influence on the well layer 32 by the spontaneous polarization ofthe second layer 42 with the high Al composition ratio Ax can besuppressed. Accordingly, the energy band characteristics can becontrolled to be a desired state. Thus, the overflow of electrons isreduced and the injection efficiency of holes to the active layer can beincreased. As a result, the light emitting efficiency is high.

Hereinafter, characteristics of examples according to this embodimentwill be described with comparative examples.

The configurations of semiconductor light emitting devices 110 a to 110c of first to third examples according to this embodiment are the sameas that of the semiconductor light emitting device 110 illustrated inFIG. 1. Thus, the description thereof is omitted.

FIRST EXAMPLE

In the semiconductor light emitting device 110 a of the first example,an intermediate layer 40 m includes of GaN which does not substantiallyincludes Al and has a thickness of 5 nm. A first layer 41 includesAl_(0.01)Ga_(0.99)N (i.e., a first Al composition ratio x1=0.01) and hasa thickness of 5 nm. The concentration of Mg in the first layer 41 isapproximately 1×10¹⁷ cm⁻³ to 1×10¹⁹ cm⁻³, which is relatively a lowconcentration. A second layer 42 includes Al_(0.2)Ga_(0.8)N (i.e., asecond Al composition ratio x2=0.2) and has a thickness of 5 nm. Theconcentration of Mg in the second layer 42 is approximately 1×10¹⁹ cm⁻³to 1×10²⁰ cm⁻³, which is relatively a high concentration.

SECOND EXAMPLE

In the semiconductor light emitting device 110 b of the second example,a first layer 41 includes Al_(0.01)Ga_(0.99)N and has a thickness of 2.5nm. The configuration other than that is the same as that of the firstexample.

THIRD EXAMPLE

In the semiconductor light emitting device 110 c of the third example, afirst layer 41 includes Al_(0.01)Ga_(0.99)N and has a thickness of 0.5nm. The configuration other than that is the same as that of the firstexample.

FIRST COMPARATIVE EXAMPLE

The configuration of a semiconductor light emitting device 119 a of afirst comparative example is the same as that of the semiconductor lightemitting device 119 of the comparative example illustrated in FIG. 5.More specifically, the semiconductor light emitting device 119 a doesnot include an intermediate layer 40 m and a first layer 41. Theconfiguration other than that is the same as that of the semiconductorlight emitting device 110 a of the first example.

More specifically, in the semiconductor light emitting device 119 a ofthe first comparative example, a second layer 42 is provided in contactwith a well layer 32 of a light emitting portion 30. The second layer 42includes Al_(0.2)Ga_(0.8)N and has a thickness of 5 nm. Theconcentration of Mg in the second layer 42 is 1×10¹⁹ cm⁻³ to 1×10²⁰cm⁻³.

SECOND COMPARATIVE EXAMPLE

A semiconductor light emitting device 119 b of a second comparativeexample has an intermediate layer 40 m, a first layer 41, and a secondlayer 42. In other words, the configuration of the semiconductor lightemitting device 119 b is similar to that of the semiconductor lightemitting device 110 illustrated in FIG. 1. However, the intermediatelayer 40 m is thin. In the semiconductor light emitting device 119 b,the thickness of the intermediate layer 40 m is 0.5 nm. Theconfiguration other than that (e.g., the configurations of the firstlayer 41 and the second layer 42) is the same as that of thesemiconductor light emitting device 110 a of the first example.

THIRD COMPARATIVE EXAMPLE

A semiconductor light emitting device 119 c of a third embodiment has anintermediate layer 40 m, a first layer 41, and a second layer 42. Inother words, the configuration of the semiconductor light emittingdevice 119 c is similar to that of the semiconductor light emittingdevice 110 illustrated in FIG. 1. However, the intermediate layer 40 mis thin. In the semiconductor light emitting device 119 c, the thicknessof the intermediate layer 40 m is 2.5 nm. The configuration other thanthat (e.g., the configurations of the first layer 41 and the secondlayer 42) is the same as that of the semiconductor light emitting device110 a of the first example.

FIG. 7 is a graph illustrating characteristics of the semiconductorlight emitting devices.

More specifically, FIG. 7 illustrates characteristics of thesemiconductor light emitting devices 110 a to 110 c of the first tothird examples and the semiconductor light emitting devices 119 a to 119c of the first to third comparative examples. In FIG. 7, the horizontalaxis represents a current If flowing through each of the semiconductorlight emitting devices and the vertical axis represents a light emittingefficiency Eff.

These semiconductor light emitting devices 110 a to 110 c and 119 a to119 c are a blue LED whose peak wavelength is 450 nm.

As shown in FIG. 7, in the semiconductor light emitting device 119 a ofthe first comparative example in which an intermediate layer 40 m is notprovided, the light emitting efficiency Eff is extremely small. It isthought that this comes from that the second layer 42 with high Alcomposition ratio Ax is in contact with the well layer 32 of the lightemitting portion 30 and the characteristics of the energy Eg of the welllayer 32 is deteriorated due to the spontaneous polarization in thesecond layer 42.

The light emitting efficiency Eff is low also in the semiconductor lightemitting device 119 b of the second comparative example in which thethickness of the intermediate layer 40 m is 0.5 nm.

In the semiconductor light emitting device 119 c of the thirdcomparative example in which the thickness of the intermediate layer 40m is 2.5 nm, the light emitting efficiency Eff increases as comparedwith the semiconductor light emitting device 119 b, but it is notsufficient.

In contrast, all the semiconductor light emitting devices of the firstto third examples show a high light emitting efficiency Eff. Since thehigh light emitting efficiency is observed even when the first layer 41has a thickness of 0.5 nm, it can be seen that a thickness ofapproximately 0.5 nm may be enough for the first layer 41 in the casewhere the thickness of the intermediate layer 40 m is approximately 5nm.

As described above, in the semiconductor light emitting device 110 (thesemiconductor light emitting devices 110 a to 110 c) according to theembodiment, the high light emitting efficiency Eff can be obtained whenthe intermediate layer 40 m has a thickness of at least approximately 5nm. When the thickness of the intermediate layer 40 m is, for example,not larger than 2.5 nm (e.g., the semiconductor light emitting devices119 b and 119 c of the second and third comparative examples), the lightemitting efficiency Eff is low.

FIG. 8A to FIG. 8C are schematic views illustrating the simulationresults of the characteristics of the semiconductor light emittingdevices.

More specifically, FIG. 8A, FIG. 8B, and FIG. 8C illustrate the resultsof simulating the energy band characteristics in the semiconductorlayers from the light emitting portion 30 to the p-type semiconductorlayer 20 in each of the semiconductor light emitting device 110 a of thefirst example and the semiconductor light emitting devices 119 b and 119c of the second and third comparative examples.

As shown in FIG. 8A, in the semiconductor light emitting device 119 b ofthe second comparative example in which the thickness of theintermediate layer 40 m is 0.5 nm, the energy characteristic of the welllayer 32 which is the closest to the p-type semiconductor layer 20 iscurved, and the energy band is bent. It is thought that this is due tothe influence of the spontaneous polarization of the second layer 42whose Al composition ratio Ax is high, for example.

As shown in FIG. 8B, also in the semiconductor light emitting device 119c of the third comparative example in which the thickness of theintermediate layer 40 m is 2.5 nm, the energy characteristic of the welllayer 32 which is the closest to the p-type semiconductor layer 20 isalso curved, and the curve of the energy band is not sufficientlyremoved.

In contrast, as shown in FIG. 8C, in the semiconductor light emittingdevice 110 a of the first embodiment in which the thickness of theintermediate layer 40 m is 5 nm, the energy characteristic of the welllayer 32 which is the closest to the p-type semiconductor layer 20 islinear, and the curve of the energy band is sufficiently removed. As aresult, a high light emitting efficiency can be obtained.

Consequently, the thickness of the intermediate layer 40 m is set to benot smaller than 3 nm in the semiconductor light emitting device 110according to the embodiment.

Hereinafter, characteristics of a light emitting efficiency Eff and avoltage Vf of semiconductor light emitting devices including the casewhere an intermediate layer 40 m is thick will be described.

FOURTH COMPARATIVE EXAMPLE

A semiconductor light emitting device 119 d of a fourth comparativeexample has an intermediate layer 40 m, a first layer 41, and a secondlayer 42. In other words, the configuration of the semiconductor lightemitting device 119 d is similar to that of the semiconductor lightemitting device 110 illustrated in FIG. 1. However, the intermediatelayer 40 m is thick. In the semiconductor light emitting device 119 d,the thickness of the intermediate layer 40 m is 10.0 nm. Theconfiguration other than that (e.g., the configurations of the firstlayer 41 and the second layer 42) is the same as that of thesemiconductor light emitting device 110 a of the first example.

FIG. 9A to FIG. 9C are graphs illustrating characteristics of thesemiconductor light emitting devices.

More specifically, FIG. 9A and FIG. 9B illustrate the light emittingefficiencies Eff and voltages Vf of the semiconductor light emittingdevice 110 a of the first example, as well as the semiconductor lightemitting devices 119 b, 119 c, and 119 d of the second, third, andfourth comparative examples. In FIG. 9A and FIG. 9B, the horizontal axisrepresents a current If flowing through each semiconductor lightemitting device. In FIG. 9A, the vertical axis represents a lightemitting efficiency Eff. In FIG. 9B, the vertical axis represents avoltage Vf. FIG. 9C illustrates a relationship between the thickness tmof the intermediate layer 40 m and a light emitting efficiency Eff inthese semiconductor light emitting devices. In FIG. 9C, the horizontalaxis represents the thickness tm of the intermediate layer 40 m and thevertical axis represents a light emitting efficiency Eff when thecurrent If is 20 mA.

As shown in FIG. 9A, the light emitting efficiency Eff is low in thecase where the thickness tm of the intermediate layer 40 m is as thin as0.5 nm and 2.5 nm like the semiconductor light emitting devices 119 band 119 c and in the case where the thickness tm of the intermediatelayer 40 m is excessively thick like the semiconductor light emittingdevice 119 d.

As shown in FIG. 9C, in the case where the thickness of the intermediatelayer 40 m is approximately not smaller than 3 nm and not larger than 8nm, the light emitting efficiency Eff is high.

As described above, the light emitting efficiency is low when thethickness tm of the intermediate layer 40 m is excessively large. It isthought that the reason is that the injection efficiency of holes to theactive layer (the light emitting portion 30) decreases when theintermediate layer 40 m is thick because the mobility of holes is smallin a nitride semiconductor.

Furthermore, as shown in FIG. 9B, in the semiconductor light emittingdevice 119 d whose intermediate layer 40 m is thick, the voltage Vf ishigh. With regard to this point also, the thickness tm of theintermediate layer 40 m is properly set.

As described above, in the semiconductor light emitting device 110according to the embodiment, the thickness of the intermediate layer 40m is not smaller than 3 nm and not larger than 8 nm.

FIG. 10A to FIG. 10C are schematic views illustrating characteristics ofsemiconductor light emitting devices according to the embodiment of theinvention.

More specifically, FIG. 10A to FIG. 10C illustrate characteristics ofother semiconductor light emitting devices 111 to 113 according to theembodiment, respectively. FIG. 10A to FIG. 10C illustrate the changes ofAl composition ratios Ax in semiconductor layers from a light emittingportion 30 to a p-type semiconductor layer 20. In FIG. 10A to FIG. 10C,the horizontal axis represents the Z-axis direction (stacked direction)and the vertical axis represents an Al composition ratio Ax in thesemiconductor layers.

As shown in FIG. 10A, the other semiconductor light emitting device 111according to the embodiment includes an intermediate layer 40 m, a firstlayer 41, and a second layer 42 and further includes a third layer 43which is provided between the first layer 41 and the second layer 42 andwhich includes AlGaN having a third Al composition ratio x3. That is,the third layer 43 includes Al_(x3)Ga_(1-x3)N, where third Alcomposition ratio x3 is an atomic ratio of Al among group III elements.The third Al composition x3 is between the first Al composition ratio x1and the second Al composition ratio x2.

As shown in FIG. 10B, the other semiconductor light emitting device 112according to the embodiment includes an intermediate layer 40 m, a firstlayer 41, a second layer 42, and a third layer 43 and further includes afourth layer 44 which is provided between the third layer 43 and thesecond layer 42 and which includes AlGaN having a fourth Al compositionratio x4. That is, the fourth layer 44 includes Al_(x4)Ga_(1-x4)N, wherethe fourth Al composition ratio x4 is an atomic ratio of Al among groupIII elements. The fourth Al composition ratio x4 is between the third Alcomposition ratio x3 and the second Al composition ratio x2.

As described above, multiple layers can be provided between theintermediate layer 40 m which does not substantially include Al and thesecond layer 42 which include Al with a high composition ratio. The Alcomposition ratio in the multiple layers increases from the intermediatelayer 40 m toward the second layer 42.

The number of the multiple layers to be provided between theintermediate layer 40 m and the second layer 42 are arbitrarily.

Furthermore, the Al composition ratio Ax may change inside one layer.

As shown in FIG. 10C, the other semiconductor light emitting device 113according to the embodiment includes an intermediate layer 40 m, a firstlayer 41, and a second layer 42 and further includes a third layer 43which is provided between the first layer 41 and the second layer 42 andwhich includes AlGaN having a third Al composition ratio x3 between afirst Al composition ratio x1 and a second Al composition ratio x2. TheAl composition ratio Ax in the third layer 43 increases along thedirection from a light emitting portion 30 to a p-type semiconductorlayer 20.

The light emitting efficiency can be improved also in the semiconductorlight emitting devices 111 to 113 having such configurations asdescribed above.

As described above, in the semiconductor light emitting devicesaccording to the embodiment, the configuration in which the Alcomposition ratio Ax is increased stepwise or continuously (that is,“increased”) from the first layer 41 to the second layer 42 is used, andthe characteristics of the changes of the Al composition ratio Ax arearbitrarily.

In the configuration of the light emitting portion 30 of thesemiconductor light emitting device 110 illustrated in FIG. 2, the lightemitting portion 30 can further include a barrier layer 31 provided on ap-type semiconductor layer 20 side of the well layer 32 which is theclosest to the p-type semiconductor layer 20. At this time, in thesemiconductor light emitting device 110 according to the embodiment, thebarrier layer 31 does not substantially include Al. In other words, thebarrier layer 31 is based on In_(z2)Ga_(1-z2)N (0≦z2<1). In this case,the total thickness of the barrier layer 31 and the intermediate layer40 m is set to be not smaller than 3 nm and not larger than 8 nm.

In addition, the intermediate layer 40 m in the semiconductor lightemitting device 110 may be considered as the barrier layer 31 providedon the p-type semiconductor layer 20 side of the well layer 32 which isthe closest to the p-type semiconductor layer 20 in the light emittingportion 30. However, the barrier layer 31 provided on the p-typesemiconductor layer 20 side of the well layer 32 which is the closest tothe p-type semiconductor layer 20 is considered as the intermediatelayer 40 m.

In this specification, “nitride semiconductors” are intended herein toinclude semiconductors with all the compositions which are obtained bychanging each of composition ratios x, y, and z in the respective rangesin the chemical formula: B_(x)In_(y)Al_(z)Ga_(1-x-y-z)N (0≦x≦1, 0≦y≦1,0≦z≦1, x+y+z≦1). Furthermore, in the above chemical formula, the“nitride semiconductors” are intended to include ones further includingthe group V elements other than N (nitride), ones further includingvarious elements which are to be added to control various propertiessuch as conductivity types, and ones further including various elementswhich are included unintentionally.

Hereinabove, exemplary embodiments of the invention are described withreference to specific examples. However, the invention is not limited tothese specific examples. For example, various modifications made by oneskilled in the art in regard to the configurations, sizes, materialqualities, arrangements, etc., of components of semiconductor lightemitting devices such as n-type semiconductor layers, p-typesemiconductor layers, light emitting portions, well layers, barrierlayers, intermediate layers, first to forth layers, electrodes,substrates, and buffer layers are included in the scope of the inventionto the extent that the purport of the invention is included.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility; and are included inthe scope of the invention to the extent that the purport of theinvention is included.

Moreover, all semiconductor light emitting devices practicable by anappropriate design modification by one skilled in the art based on thesemiconductor light emitting devices described above as exemplaryembodiments of the invention also are within the scope of the inventionto the extent that the purport of the invention is included.

Furthermore, various modifications and alterations within the spirit ofthe invention will be readily apparent to those skilled in the art. Allsuch modifications and alterations should therefore be seen as withinthe scope of the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modification as would fall within the scope andspirit of the inventions.

The invention claimed is:
 1. A semiconductor light emitting device,comprising: an n-type semiconductor layer including a nitridesemiconductor; a p-type semiconductor layer of a nitride semiconductor;a light emitting portion provided between the n-type semiconductor layerand the p-type semiconductor layer and including a plurality of quantumwell layers, each of the quantum well layers including a nitridesemiconductor; a first intermediate region provided between the lightemitting portion and the p-type semiconductor layer, the firstintermediate region including: a first region includingAl_(x1)Ga_(1-x1)N, wherein in the first region a first Al compositionratio x1 is an atomic ratio of Al among group III elements, the first Alcomposition ratio x1 being not lower than 0.001 and not higher than0.05, the first region having a thickness not thinner than 0.5nanometer, and a second region provided between the first region and thep-type semiconductor layer, the second region contacting the p-typesemiconductor layer, the second region including Al_(x2)Ga_(1-x2)N,wherein in the second region a second Al composition ratio x2 is anatomic ratio of Al among group III elements, the second Al compositionratio x2 being not lower than 0.1 and not higher than 0.2, aconcentration of a p-type impurity in the second region being higherthan a concentration of a p-type impurity in the first region; and asecond intermediate region provided between the first region and thelight emitting portion, the second intermediate region contacting one ofthe quantum well layers, the second intermediate region having athickness not thinner than 3 nanometers and not thicker than 8nanometers and including In_(z1)Ga_(1-z1)N, wherein in the secondintermediate region an In composition ratio z1 is an atomic ratio of Inamong group III elements, the In composition ratio z1 being not lowerthan 0 and lower than
 1. 2. The device according to claim 1, furthercomprising a third region provided between the first region and thesecond region, the third region including Al_(x3)Ga_(1-x3)N, wherein inthe third region a third Al composition ratio x3 is an atomic ratio ofAl among group III elements, the third Al composition being between thefirst Al composition ratio x1 and the second Al composition ratio x2. 3.The device according to claim 2, further comprising: a fourth regionprovided between the third region and the second region, the fourthregion including Al_(x4)Ga_(1-x4)N, wherein in the fourth region afourth Al composition ratio x4 is an atomic ratio of Al among group IIIelements, the fourth Al composition ratio x4 being between the third Alcomposition ratio x3 and the second Al composition ratio x2.
 4. Thedevice according to claim 2, wherein the third Al composition ratio x3in the third region increases along a direction from the light emittingportion toward the p-type semiconductor layer.
 5. The device accordingto claim 4, wherein the third region includes a plurality of layers. 6.The device according to claim 1, wherein an Al composition ratioincreases from the first region toward the second region.
 7. The deviceaccording to claim 1, wherein the first Al composition ratio x1 issubstantially 0.05 and a thickness of the first region is substantially5 nanometers.
 8. The device according to claim 1, wherein the second Alcomposition ratio x2 is substantially 0.20 and a thickness of the secondregion is substantially 5 nanometers.
 9. The device according to claim1, wherein the second intermediate region includes substantially no Al.10. The device according to claim 1, wherein the second intermediateregion is made of one of GaN and InGaN.
 11. The device according toclaim 1, wherein the light emitting portion includes a plurality ofbarrier layers, the barrier layers and the quantum well layers beingalternately stacked along a direction from the n-type semiconductorlayer toward the p-type semiconductor layer.
 12. The device according toclaim 11, wherein one of the plurality of barrier layers is in contactwith the n-type semiconductor layer.
 13. The device according to claim1, wherein the p-type semiconductor layer is of made of GaN.
 14. Thedevice according to claim 1, wherein the n-type semiconductor layerincludes an n-type GaN layer and a n-type guide layer, the n-type guidelayer is provided between the n-type GaN layer and the light emittingportion, and the n-type guide layer is made of one of GaN including Siand InGaN including Si.
 15. The device according to claim 1, wherein thep-type semiconductor layer includes a p-type GaN contact layer and ap-type GaN layer provided between the p-type GaN contact layer and thesecond region, and a concentration of a p-type impurity included in thep-type GaN layer is lower than a concentration of a p-type impurityincluded in the p-type GaN contact layer.
 16. The device according toclaim 1, further comprising: a substrate made of sapphire, the n-typesemiconductor layer being provided between the substrate and the lightemitting portion; and a buffer layer provided between the substrate andthe n-type semiconductor layer.
 17. The device according to claim 1,further comprising: an n-side electrode provided in contact with then-type semiconductor layer; and a p-side electrode provided in contactwith the p-type semiconductor layer.